Post-lithography misalignment correction with shadow effect for multiple patterning

ABSTRACT

Misalignment created during a multiple-patterning process is a serious challenge for critical dimension (CD) control and layout design in continuing integrated-circuit device scaling. A number of post-lithography misalignment correction technologies based on the shadow effect are invented for multi-pattering lithographic applications. When applied to transfer patterns from a top layer to an underneath layer, the subtractive shadow effect in anisotropic plasma etching combined with a hard-mask process, will shift the position of features such that the previously produced misalignment can be corrected. Also, the additive shadow effect in a sputtering/evaporation process can be used to correct the misalignment. Several designs of plasma etching and sputtering equipment are proposed to achieve the local control of correction to reduce the non-uniformity of misalignment. Specifically, these misalignment correction methods allow the semiconductor industry to print sub-32 nm (half-pitch) features using the double-patterning technique with currently existing lithographic tools (e.g., 193-nm DUV scanner), therefore postponing the need of expensive next-generation lithography (NGL) such as EUV (extreme ultraviolet, wavelength: 13.5 nm), nano-imprint, and electron-beam direct write technologies. In general, the developed misalignment correction methods can be applied to all the existing lithography technologies to print features smaller than their physical resolution limits in a production-worthy manner.

Optical DUV (deep ultraviolet, 193 nm) immersion lithography with NA’1.3 has the capability of printing half-pitch features down to about 40 nm. The potential next-generation lithography (NGL) technologies include EUV (extreme ultraviolet), maskless, and nano-imprint lithography [1]. However, all these NGL technologies face their own technological challenges and still need a long development time before their applications in high-throughput manufacturing. Recently, double patterning has attracted much industrial interest which prints less dense line/space or contact hole patterns twice on the same wafer to finally obtain dense patterns with double spatial frequency [2]. In principle, a similar multiple-patterning concept can be developed but both need extremely high alignment accuracy. It is a severe challenge to significantly reduce the misalignment budget in a lithographic process to meet the multiple-patterning requirement at 32 nm half pitch and below. Therefore, post-lithography misalignment correction will be a promising alternative technology enabling multiple patterning (including double patterning) for future semiconductor manufacturing. It provides a production-worthy method for the whole semiconductor industry to continue device scaling beyond sub-40 nm generation with no need of NGL technology.

Several post-lithography misalignment correction techniques based on the shadow effect in anisotropic plasma etch or sputtering/evaporation processes are invented which allow us to significantly reduce the misalignment created in a lithographic process. Next, we demonstrate the forming mechanism of misaligned dense line/space patterns during a double-patterning process. Similar misalignment mechanism exists in the multiple-patterning process.

In FIG. 1, the cross-section view of a double-patterning process flow to print misaligned dense line/space and contact hole patterns is shown. Starting with a targeted layer on top of the substrate, semi-dense features are printed on the targeted layer using standard lithographic and dry etching processes (in step (2)), with their pitch size twice the size of desired pitch as shown in FIG. 1(5). After that, another resist film is spun on the top and the wafer is exposed again with the same pattern shifted by a distance such that ideally every trench center in the resist will be coincident with the center of corresponding line structure underneath. However, as shown in FIG. 1(3), the trench is not located right at the center of the line (e.g., A≠B) due to an unavoidable misalignment during a lithographic process. Consequently, this misalignment will be transferred to the targeted layer underneath in the following plasma etching process assuming its etching direction is vertical as shown in step (4). This results in unequal width of the final dense features (e.g., C≠D) and brings serious challenges to process yield and circuit design/layout. Similar problem exists in a multiple-patterning process to obtain dense features with their pitch reduced to ⅓, ¼, . . . , of the original size corresponding to the resolution limit of a conventional lithographic tool.

The process flow shown in FIG. 2 is similar to previously described process except that a sacrificial layer is deposited before the second lithographic step. This sacrificial layer finally will be released and can be polished to flatten the surface (e.g., with a CMP process) resulting in an improved lithographic process window. However, if the plasma etching direction is vertical, the misalignment is still transferred to the targeted layer as shown in FIG. 2(6) (e.g., C≠D).

As demonstrated before, if the ions' incident direction is vertical to the substrate surface, the misalignment will be transferred to the targeted layer. Next, we shall describe a so-called shadow effect which occurs in both subtractive anisotropic plasma etching and additive sputtering/evaporation processes, and can be used to correct the misalignment created during a lithographic process. As shown in FIG. 3, we compare two plasma etching processes in which one etching is in vertical direction (1), and the other etching is tilted by certain angle (2). The tilted etching can be achieved by either directly tilting the ions' bombarding direction or tilting the wafer surface. Both dash lines drawn in the figure represent the central position of the right-side structure formed in a vertical etching process. When the etching direction is tilted as shown in (2), two effects occur. First, the slope of left side walls of the etched structures becomes more vertical while the opposite happens to the slope of right side walls, which results in asymmetric final structures. Secondly, relative to the dashed line which indicates the center of a final structure produced in the vertical etching, the mass center of the asymmetric structure formed in a tilted etching process moves toward the right side. The shift distance depends on the etching direction as well as the etching selectivity. Even though the etched structures are asymmetric, this asymmetry will not transfer to the substrate if we can use those structures as a hard mask with high selectivity when etching the substrate.

The shadow effect also occurs in an additive process such as sputtering or evaporation deposition as shown in FIG. 4. First, a dense structure is patterned and etched into the substrate. When the sputtering direction is tilted toward the right side, the step coverage is not conformal as deposition occurs only on the top and left side walls. This effect can also be used to correct misalignment.

The misalignment correction process can vary depending on whether a sacrificial layer is used between two exposures (e.g., in a double-patterning process) and when the misalignment can be measured. We shall demonstrate the correction processes based on the shadow effect of anisotropic plasma etching without a sacrificial layer first.

If the misalignment can be measured right after the exposed resist is developed and baked, then the correction process is shown in FIG. 5. The tilting angle can be arbitrary (2-D) and should be adjusted according to measured values of misalignment in both X and Y directions; but here we only use the right-tilted etching as an example to demonstrate the concept. First, we put a hard mask on top of the substrate and pattern this hard-mask layer with semi-dense features wherein the size of lines is three times of the size of spaces as shown in FIG. 5(1) and (2). In step (3), resist is spun on the patterned hard-mask layer and exposed again to print another pattern of semi-dense features with its position shifted. Ideally, we would like to shift the second pattern in such a way that the trenches in the resist will be located right at the center of the corresponding line structures underneath. However, due to misalignment in a lithographic process, perfect overlay can not be achieved and as a result unequal distances (e.g., C<B) are created as shown in FIG. 5(3). To correct the resultant misalignment in this condition, the etching direction will be tilted toward the right side as shown in step (4). Owing to the shadow effect as demonstrated before, the tilted etching can produce a dense structure with uniform bottom CD (critical dimension, e.g., =A as shown in FIG. 5(4)) everywhere. Even the slope of final dense line/space structure is not uniform, due to the high selectivity of hard mask to the substrate in the following plasma etching, this non-uniformity will not be transferred to the substrate or only little effect will be seen after an anisotropic dry etching.

Another possibility exists if a sacrificial (to be used as a hard-mask) layer is deposited between two lithographic exposures as shown in FIG. 6(3). In this case, misalignment can be corrected with a similar tilted etching when the pattern on the resist is transferred to the hard-mask layer. First, the targeted layer is patterned with semi-dense features wherein the size of lines is three times of the size of spaces. Then a hard-mask layer is deposited and an optional CMP step may be used to planarize the surface. After this, a resist is spun on the top and the second lithographic step will print another pattern of semi-dense features which are the same as the previous pattern and ideally should be shifted by a distance equal to the space size. Due to the misalignment in the second exposure, the shift distance will not be exactly equal to the desired value. As a result, the distance B indicated in FIG. 6(4) is not equal to C. Apparently, C-B (assuming C>B) is the misalignment which can be measured right after the lithographic step. After the misalignment data is available, the tilted etching will be applied to correct this error during the following dry etching process. The goal is to create equal line/space pattern (e.g., size=A as shown in step (7)) with double spatial frequency. Ideally, if the hard-mask structure such as slope and CD does not change during the following etching, then the bottom CDs as indicated in FIG. 6(5) should be equal ( i.e., A′=A) such that the transferred line/space structures in the targeted layer have the uniform size “A” everywhere. Practically, however, the shape of hard-mask structures may slightly change after dry etching; therefore, A′ will be close, but not exactly equal to A. Similar conclusion about this small difference is applicable to all the cases discussed in this patent, but not mentioned repeatedly elsewhere.

On the other hand, if misalignment can not be measured right after the lithographic step, then we will not be able to apply the shadow effect to correct the misalignment during the first following etching. In this condition, the first anisotropic etching direction will still be vertical as shown in FIG. 7(4) due to the lack of real-time data for correction. After this plasma etching, the misalignment in the resist pattern will be transferred to the hard mask layer underneath (e.g., A≠B). If misalignment can be measured only after the first etching (with patterned resist on the top) is finished, then this hard mask layer is necessary as we shall apply the tilted etching to correct the misalignment during the second etching which will transfer the pattern on the hard mask to the targeted layer as shown in step (5).

All we discussed before is to use the subtractive shadow effect of a tilted anisotropic etching to correct the misalignment created during the lithographic processes. Another possibility is to apply the additive shadow effect of a sputtering/evaporation as demonstrated before to correct the misalignment. As shown in FIG. 8(1) and (2), the targeted layer is deposited on top of the substrate and patterned with semi-dense features first. A sacrificial material is then deposited to fill the trenches and may be polished (with a CMP process) to planarize the surface. A resist thin film is spun on the top and another pattern of semi-dense features is printed on the resist, however, with a misalignment such that unequal distances are created in the following vertical dry etching (e.g., A≠B as shown in (4)). After that, the misaligned pattern is etched into the targeted layer as shown in (5). A following isotropic etching (either wet or dry) will be used to produce the undercut structures as shown in (6) assuming the shape of the top sacrificial layer is not changed. However, this assumption is not essential and a slight change of the shape of top sacrificial layer due to the isotropic etching can be allowed. This undercut forming step is important in that it will remove the undesired effect of shrinking the trench in the following sputtering/evaporation process. The misalignment correction is then achieved by tilting the sputtering angle (toward the left side in this example) such that only the top and the left side wall are covered by the sputtering material. In reality, the tilting angle can be arbitrary (2-D) depending on the misalignment in both X and Y directions. In this way, the unequal width of line structures as seen in FIG. 8(7) can be compensated such that uniform line CD as shown in (8) can be obtained. In step (9), a CMP process is applied to remove the top layers and expose the sacrificial layer which will be etched away in step (10). The final line/space structures after sacrificial release with double spatial frequency is shown in FIG. 8(10).

In all above discussions, we assume that misalignment is uniform for every field on the wafer. However, practically each field might have different misalignment (in both X and Y); therefore, it is valuable to develop local correction technology which allows us to correct varying misalignment on different fields in the same etching process. There are two possible ways to achieve this. The first way is to divide the bottom (or top/both) electrode of etching chamber to a number of smaller areas and each of them is electrically isolated from each other such that different voltages can be applied to different areas. In this way, we are able to control the voltages applied to different areas on the same wafer and adjust the ions' speed for different areas during the same plasma etching. Since the correction is affected by the local ion speed, non-uniformity of misalignment can be significantly reduced. The second way is to locally adjust the etching angle, which needs the capability to locally adjust the angles of emitting surface of electrodes. For this purpose, electrodes can be designed to consist of a number of smaller pieces of metals and each of them can be moved (up and down) and rotated independently.

For sputtering/evaporation-based local correction of non-uniform misalignment, the local control of incident speed and angle of sputtering atoms can be achieved by inserting a speed/angle filter (which can determine the local incident speed and angle of atoms) between sputtering source and wafer/substrate.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts a process example to demonstrate how misalignment is produced in a double-patterning process.

FIG. 2 depicts another process example (wherein a sacrificial layer is used) to demonstrate how misalignment is produced in a double-patterning process.

FIG. 3 depicts the shadow effect (not to scale) in an anisotropic plasma etching process.

FIG. 4 depicts the shadow effect (not to scale) in a sputtering/evaporation process.

FIG. 5 depicts an anisotropic etching-based process (without a sacrificial layer) to correct the misalignment created during the lithographic step and measured right after that lithographic step.

FIG. 6 depicts an anisotropic etching-based process to correct the misalignment created during the lithographic step and measured right after that lithographic step.

FIG. 7 depicts an anisotropic etching-based process to correct the misalignment created during the lithographic step and measured right after the first etching step.

FIG. 8 depicts a sputtering-based process to correct the misalignment created during the lithographic step.

REFERENCES

-   [1] International Technology Roadmap for Semiconductors (ITRS), 2005     version. -   [2] Yan Borodovsky, “Marching to the Beat of Moore's Law”, plenary     talk in SPIE Microlithography, San Jose, Calif., 2006. 

1. A tilted anisotropic plasma etching process is shown in FIG. 1 that can be achieved by either directly tilting the ions' bombarding direction or tilting the wafer surface. It can be used to correct the misalignment created during the lithographic step and measured right after that lithographic step, the process comprising: a. A hard-mask layer with high etching selectivity to the substrate is put on top of the substrate as shown in FIG. 1(1), and first semi-dense features are printed with a standard lithographic process and etched into the hard-mask layer wherein the size of lines is three times of the size of spaces as shown in and FIG. 1(2). b. In step (3), resist is spun on the hard-mask layer and exposed again to print another pattern of semi-dense features with its position shifted. Ideally, the second pattern should be shifted by a distance equal to the space size such that every trench in the resist will be located right at the center of the corresponding line structure underneath. However, such perfect overlay practically is impossible due to the misalignment in the second lithographic process and unequal distances (e.g., C<B) are created as shown in FIG. 1(3). c. The misalignment is measured right after the second lithographic step (3), based on which we can calculate the needed tilting direction and etching angle to correct the misalignment. d. The shadow effect in an anisotropic plasma etching will be applied to correct the resultant misalignment wherein the etching direction is tilted (for example, toward the right side) as shown in step (4). Due to the shadow effect, the tilted etching can produce dense structures with equal bottom width everywhere. e. Even the slope of final dense line/space structures is not uniform, due to the high selectivity of hard mask to the substrate in the following plasma etching, this non-uniformity will not be transferred to the substrate or only little effect will be seen. Therefore, well-aligned line/space substrate structures can be achieved during the plasma etching process to transfer the hard-mask pattern to the substrate.
 2. A method similar to claim 1, except that a sacrificial layer is deposited between two lithographic exposures as shown in FIG. 2(3). In this case, misalignment can be corrected with a similar tilted etching when the pattern on the resist is transferred to the hard-mask layer. a. First, a pattern of semi-dense features is printed on the targeted layer wherein the size of lines is three times of the size of spaces. b. A hard-mask layer is then deposited and an optional CMP step may be used to planarize the surface. c. A resist is spun on the top and the second lithographic step will print another pattern of semi-dense features which is the same as the previous pattern but ideally to be shifted by a distance equal to the space size. d. Due to the misalignment in the second exposure, the shift distance will not be exactly equal to the desired value. As a result, the distance indicated in FIG. 2(4) B is not equal to C; and C-B (assuming C>B) is the misalignment which can be measured right after the lithographic step. e. After the misalignment measurement data is available, the tilted etching is applied to correct it during the following dry etching process to create an equal-line/space pattern (e.g., size=A as shown in step (7)) with double spatial frequency. Ideally, if the hard-mask structure such as slope and CD does not change during the following etching, then the bottom CDs as indicated in FIG. 2(5) will be equal ( i.e., A′=A) such that the transferred structures in the targeted layer have the uniform line/space size “A” everywhere. Practically, however, the shape and CD of hard-mask structures may slightly change after the dry etching; therefore, A′ will be close, but not exactly equal to A. f. This small difference (A-A′) can be compensated when calculating the tilting angle of the plasma etching though.
 3. If misalignment can not be measured right after the lithographic step, then we will not be able to apply the shadow effect to correct the misalignment during the first dry etching whose etching direction therefore will still be vertical as shown in FIG. 3(4) due to the lack of real-time data for correction. The process is similar to that of claim 2 except: a. After the first plasma etching (with resist on the top), the uncorrected misalignment in the resist pattern is transferred to the hard mask layer underneath (e.g., A≠B). b. Misalignment is measured after the first anisotropic etching process is finished. c. Based on the measured misalignment data, the tilted etching will be applied to correct the misalignment during the second etching which will transfer the pattern on the hard mask to the targeted layer as shown in FIG. 3(5).
 4. Another possibility is to apply the additive shadow effect of a sputtering/evaporation to correct the misalignment, the process comprising: a. As shown in FIG. 4(1) and (2), the targeted layer is deposited on top of the substrate and a pattern of semi-dense features is printed first with a standard lithographic process. b. A sacrificial material is then deposited to fill the trenches and may be polished (with a CMP process) to planarize the surface. c. A resist thin film is spun on the top and another pattern of semi-dense features is printed on the resist, however, with a misalignment such that unequal distances are created in the following vertical dry etching (e.g., A≠B as shown in (4)). d. The misaligned pattern is etched into the targeted layer as shown in (5). e. A following isotropic etching (either wet or dry) will be used to produce the undercut structures as shown in (6) assuming the shape of the top sacrificial layer is not changed. However, this assumption is not essential and a slight change of the shape of the top sacrificial layer due to the isotropic etching can be allowed. This undercut forming step is important in that it will remove the undesired effect of shrinking the trench in the following sputtering/evaporation process. f. The misalignment is corrected by tilting the sputtering angle (to the left side in this example) such that only the top and the left side wall are covered by the sputtering material. Therefore, the unequal width of line structures as seen in FIG. 4(7) can be compensated to get uniform line CD as shown in (8). g. A following CMP process removes the top layers and exposes the sacrificial layer. h. The sacrificial layer is etched away in step (10) to form the final line/space structures with double spatial frequency as shown in FIG. 4(10).
 5. Practically each field on the wafer might have different misalignment (in both X and Y); therefore, local correction technology which allows us to correct varying misalignment on different fields in the same etching process is needed. There are several possible ways to achieve this: a. The first way is to divide the bottom (or top/both) electrode of etching chamber to a number of smaller areas and each of them is electrically isolated from each other such that different voltages can be applied to different areas. In this way, we are able to control the voltages applied to different areas on the same wafer and adjust the ions' speed for different areas during the same plasma etching. Since the correction is affected by the local ion speed, non-uniformity of misalignment can be significantly reduced. b. The second way is to locally adjust the etching angle, which needs the capability to locally adjust the angles of emitting surface of electrodes. For this purpose, electrodes can be designed to consist of a number of smaller pieces of metals and each of them can be moved (up and down) and rotated independently.
 6. For sputtering/evaporation-based local correction of non-uniform misalignment, the local control of incident speed and angle of sputtering atoms can be achieved by inserting a speed/angle filter (which can determine the local incident speed and angle of atoms) between sputtering source and wafer/substrate. 